High Concentration Layer Research Articles

Wind effect on sediment suspensions over silt-dominated mixtures: An experimental study

A series of laboratory experiments have been conducted to explore the wind effect on Sediment Suspended Concentrations (SSCs) in fine-grained coastal systems. The paddle waves were overplayed with surface-blowing winds in a wind-wave flume to mimic offshore swells coupling with local wind waves during rough weather. Both SSCs and water turbulences under different wind and wave conditions have been investigated over two kinds of sediment beds (Sediment 1, S1, D50 = 52µm and Sediment 2, S2, D50 = 90µm). The High Concentration Layers (HCL) were formed under most of the wave-only conditions, while with the introduction of the stronger wind, more sediment suspensions were transported upward, increasing SSCs in upper water elevations. The finer sediment S1 is easier to suspend than S2 under the same conditions. The enhancement of the vertical turbulence intensity (σw) by winds is the main reason for the increase in SSCs. Meanwhile, because the wind-induced turbulence can hardly penetrate the HCL, the turbulence intensities outside the HCL can be further amplified compared to the experiment without a sediment bed. The wind contributes over 65% of the SSC enlargement above the HCL under a wind of 10m/s for S1, while less than 20% inside the HCL in most wind conditions. The sediment mixing coefficient (ϵs), a crucial parameter for suspended sediment modeling, was enhanced with stronger winds. Although the existing formulas for the vertical distribution of ϵs are valid under both wave-only and small winds (2.5 m/s) for both sediment beds, the enhancement of ϵs caused by strong winds cannot be captured, requiring further research.

Modelling and analysis on high sediment concentration layer of fine sediments under wave-dominated conditions

Experiments and field observations have revealed that when silt and very fine sand are subject to oscillatory wave motion, a high shear flow layer and a high concentration layer (HCL) exist near the bottom. The behavior of the HCL is still under researched. Firstly, an intra-wave process based 1DV model was established for fine sediment transport under the combined action of waves and currents. Some key processes that were included in the model are represented through approaches for different bed forms (rippled bed and 'flat bed'), hindered settling, stratification, reference concentration and critical shear stress. A number of experimental datasets were collected to verify the model, which shows that the model is able to properly simulate the flow and sediment dynamics. Secondly, sensitivity analyses were carried out on some factors which would impact the suspended sediment concentration (SSC) profile of the HCL by the 1DV model, such as bed forms, flow dynamics, stratification effects, mobile bed effects and hindered settling. Results show that bed forms play a significant role in the HCL and determination of the shape of the concentration profile. When a current is imposed, the SSC profiles become smoother; however, sediment concentration in the lower HCL is still dominated by the wave motions. For finer sediment, the stratification effects and the mobile bed effects strongly impact the HCL. In conclusion, this paper provides a tool for the study of the HCL and an evaluation of several impact factors on the HCL.

Experiment inspired numerical modeling of sediment concentration over sand–silt mixtures

A series of flume experiments has been conducted to investigate sediment transport of sand–silt mixtures in both wave-only and wave-with-current conditions. Two types of sediments collected from a typical silty tidal flat were used: a silt-sized mixture with median grain size of 46μm, and a very fine sand-sized mixture with median grain size of 88μm. A high concentration layer (HCL) was observed near the bottom together with ripples under wave-only conditions. Sediment concentrations inside the HCL are quasi-stationary with the bulk Richardson number approaching a constant value. The thickness of the HCL can be scaled with approximately two times the damped wave boundary layer thickness. For the concentration profiles, we find that the vertical profile of the silt concentration appears different from the profile of the sand concentration, since the silt concentration decreases logarithmically within HCL, while homogeneously distributes outside the HCL. Finally, the reference concentration formulation of van Rijn (2007b) was recalibrated for the silt classes and applied in a multi-fraction model to predict the vertical concentration profile for silt and sand classes. The results show a promising agreement with the measurements, for both wave-only and wave-with-current conditions.